Typically, a voltage-fed inverter having a filter capacitor in the DC link section performs a constant output voltage control for reducing fluctuation of the output voltage due to fluctuation of the DC input voltage. FIG. 1 shows a schematic configuration of a conventional inverter device for performing the constant output voltage control. Note that the control system is shown simplified in FIG. 1. While an output current detection section (160) is provided between an inverter section (140) and a motor (150), the configuration may be any configuration as long as the current output to the motor (150) can be detected, and it may for example be a configuration where a shunt resistor is provided in the DC link section so as to detect the current.
The inverter device of FIG. 1 includes an inverter control section (200) which is a control section. The inverter control section (200) performs a correction by a voltage correction section (202) so that the output voltage is not influenced by the fluctuation of the DC voltage input to the inverter section (140). Specifically, the DC voltage value input to the inverter section (140) is detected by a DC voltage detection section (210) and given to the voltage correction section (202). The voltage correction section (202) performs a voltage correction by dividing the voltage command value by the DC voltage value (see, for example, conventional examples of Patent Documents 1 and 2).
A PWM calculation section (203) calculates a control signal for PWM control of the inverter section (140) based on the voltage command value from the voltage correction section (202), and the switching element of the inverter section (140) is turned ON/OFF in response to the control signal.
FIG. 1 shows a control system which assumes a synchronous motor as the motor (150). The control of the synchronous motor is typically performed based on a motor model which has been coordinate-converted onto the d-q coordinate system. [Expression 1] shows the equation of state of the synchronous motor which has been coordinate-converted onto the d-q coordinate system.νd=(R+sLd)id−ωLqiq νq=(R+sLd)iq+ωLdid+ωφa  [Expression 1]
A speed controller (204) and a current controller (206) perform PI control, for example. [Expression 2] shows the transfer function of the current controller (206) where PI control is performed.
                                          v            d            *                    =                                                                                          K                    id                                    ⁡                                      (                                          1                      +                                              1                                                                              T                            id                                                    ·                          s                                                                                      )                                                                    ︸                                      PI                    ⁢                                                                                  ⁢                    COMPENSATOR                                                              ⁢                              (                                                      i                    d                    *                                    -                                      i                    d                                                  )                                      -                                          ω                ⁢                                                                  ⁢                                  L                  q                                ⁢                                  i                  q                                                            ︸                                                      NON                    ⁢                                          -                                        ⁢                    INTERFERING                                    CONTROL                                                                    ⁢                                  ⁢                              v            q            *                    =                                                                                          K                    iq                                    ⁡                                      (                                          1                      +                                              1                                                                              T                            iq                                                    ·                          s                                                                                      )                                                                    ︸                                      PI                    ⁢                                                                                  ⁢                    COMPENSATOR                                                              ⁢                              (                                                      i                    q                    *                                    -                                      i                    q                                                  )                                      +                                                            ω                  ⁢                                                                          ⁢                                      L                    d                                    ⁢                                      i                    d                                                  +                                  ωϕ                  a                                                            ︸                                                      NON                    ⁢                                          -                                        ⁢                    INTERFERING                                    CONTROL                                                                                        [                  Expression          ⁢                                          ⁢          2                ]            
For example, a current vector controller (205) performs id*=0 control, maximum torque control, flux-weakening control, etc. (see, for example, Non-Patent Document 1).
Typically, the control band of a current control system formed by the current controller (206) is set to be greater than that of a speed control system formed by the speed controller (204). With a driving motor of a compressor, for example, it is often the case that the control band of the speed control system is set to about 10 Hz and the control band of the current control system to about 200 Hz. Arithmetic operations that need to be controlled at higher speed need to be performed without the intervention of the speed control system or the current control system, and the voltage correction operation by the voltage correction section (202) shown in FIG. 1 is one of these operations.
Now, the voltage correction performed by the voltage correction section (202) will be described in detail.
[Expression 3] shows the output voltage (average voltage value) V− in a case where the inverter section (140) is PWM-controlled with a carrier period T and a pulse width τ under the DC input voltage VDC (see FIG. 2).
                              V          _                =                              1            T                    ⁢                      (                                          V                DC                            ×              τ                        )                                              [                  Expression          ⁢                                          ⁢          3                ]            
The pulse width τ is obtained by [Expression 4] so that the output voltage V− and the output command voltage V* are equal to each other irrespective of the DC input voltage VDC. In this expression, by dividing V* by VDC, a voltage correction is performed so that the output voltage V− does not fluctuate (herein, the operation of dividing by VDC is referred to as a voltage correction). With such an operation, it is possible to reduce the fluctuation of the output voltage V− to the fluctuation of VDC.
                    τ        =                                            V              *                                      V              DC                                ⁢          T                                    [                  Expression          ⁢                                          ⁢          4                ]            
Here, assuming that the value of the current input to the inverter section (140) is IDC, the input power Pin is as follows.Pin=VDCIDC 
The output power Pout of the inverter section (140) is controlled at a constant value through the voltage correction by the voltage correction section (202), and the following holds.
Pin=Pout=P (constant) [note that loss at inverter section (140) is ignored]
Thus, the input current is IDC=P/VDC.
Therefore, the input voltage VDC and the input current IDC of the inverter section (140) are in such a relationship that IDC decreases as VDC increases, and IDC increases as VDC decreases. That is, where the constant output voltage control is performed, the inverter section (140), as viewed from the input side, exhibits characteristics of a negative resistor.
On the other hand, an LC filter including a reactor (120) and a capacitor (130) may undergo a phenomenon (link resonance) where it resonates at a resonance frequency f0 shown in [Expression 5].
                              f          0                =                  1                      2            ⁢            π            ⁢                          LC                                                          [                  Expression          ⁢                                          ⁢          5                ]            
In order to reduce the link resonance, a control needs to be performed where the output of the inverter section (140) is increased so as to reduce the increase of VDC when VDC is increasing whereas the output of the inverter section (140) is reduced so as to reduce the decrease of VDC when VDC is decreasing, i.e., a control such that IDC increases when VDC increases whereas IDC decreases when VDC decreases. However, if the constant output voltage control described above is performed, the link resonance cannot be reduced because a correction is performed by the voltage correction section (202) in the opposite direction to reducing the link resonance (VDC increase→IDC decrease, VDC decrease→IDC increase).